UARTs
LH79524/LH79525 User’s Guide
16-14
Version 1.0
16.3.2.6 Fractional Baud Rate Divisor Register (UARTFBRD)
UARTFBRD is the fractional portion of the baud rate divisor value.
16.3.2.6.1 Calculating the Divisor Value
The following example shows how to program a divisor value. This example assumes that
the required baud rate is 460,800 and the UARTCLK = 11.2896 MHz.
1.
Baud Rate Divisor = (11.2896 × 10
6
) ÷ (16 × 460,800) = 1.53125
2.
Integer part, i = integer (1.53125) = 1
3.
Fractional part, m = integer ((0.53125 × 64) + 0.5) = 34
4.
Generated baud rate divider = 1 + 34/64 = 1.53125
5.
Generated baud rate = (11.2896 × 10
6
) ÷ (16 × 1.53125) = 460,800
The maximum error using a 6-bit UARTFBRD Register = 1/64 × 100 = 1.56%. This occurs
when m = 1 and the error is cumulative over 64 clock ticks. Using an 11.2896 MHz crystal
produces the standard baud rates with no error.
16.3.2.6.2 Typical Baud Rates and Their Corresponding Divisor
Table 16-18 shows typical baud rates and their corresponding divisor using an example
UART clock frequency of 11.2896 MHz.
Table 16-16. UARTFBRD Register
BIT
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
FIELD
///
RESET
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
RW
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
BIT
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
FIELD
///
FRAC
RESET
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
RW
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RW
RW
RW
RW
RW
RW
ADDR
UART 0: 0xFF 0x028
UART 1: 0xFF 0x028
UART 2: 0xFF 0x028
Table 16-17. UARTFBRD Fields
BIT
NAME
DESCRIPTION
31:6
///
Reserved
Reading returns 0. Write the reset value.
5:0
FRAC
Fractional Baud Rate Divisor
This value is used with the Integer Baud Rate Divisor to
program the baud rate for the UART. See Section 16.3.2.6.1 and Section 16.3.2.5.
Table 16-18. Bit Rates and Their Corresponding Divisors
UART CLK (MHz) BAUD RATE
INTEGER DIVISOR
(UARTIBRD)
FRACTIONAL DIVISOR, m
(UARTFBRD)
11.2896
921,600
0
49
11.2896
460,800
1
34
11.2896
230,400
3
4
11.2896
115,200
6
8
11.2896
76,800
9
12